Low power wireless data communications networks are used in a number of environments in which it is either impractical to run cabling for the local area network to fixed terminal sites, or where there is a need for mobile terminals that are in communications with the network. Examples of the former include rail yards and other facilities spread over a large area. In the latter category falls the most common use of such systems, normally warehouses for stocking goods for shipment or accepting parts to be used in a manufacturing process. A typical system includes multiple base stations inter-connected by cabling to a network controller that processes the data received over the network and controls the transmission of data back to remote terminals through the base stations. A typical arrangement is one in which the network controller communicate with the host via serial data links employing twisted pair cabling.
The base stations communicate via wireless radio frequency link with a plurality of remote terminals that are typically either hand held or mounted on a vehicle such as a fork lift, cart or truck. Typical terminals include keyboards and displays and some are equipped with scanning bar code readers. The networks may be used as part of any underlying system for operation of the facility in which they are used, such as assistance in taking inventory, filling orders, directing employees as to the placement of inbound products in the warehouse space and the like. Information from the remote terminals is normally entered via key pad or bar code reader. It may indicate that an assigned task has been completed, that problems have been encountered, such as the absence of sufficient goods of a particular type to fill a particular order, and so forth.
Any application that the user wishes to implement, that can be handled by the host computer, can be implemented by using the wireless data communications network. For example, the host computer can determine minimum length paths for filling particular orders and transmit to a remote terminal located on a transportation device in a warehouse the sequence in which items should be selected for filling a particular order. The systems are also very useful in environments in which just in time parts inventory systems are implemented for prioritizing movement of materials from a receiving dock to locations in a warehouse or manufacturing facility at which they are needed.
Largely by regulatory constraint in the United States of America, these systems are typically operated at a low power level since the authorized radio transmissions for this purpose are in shared portions of the electromagnetic spectrum under the regulatory scheme implemented by the United States Federal Communications Commission (FCC). Since 1985, the FCC has approved the use of low power non-license Business Radio Systems regulated under Sub-part D of Part 15 of Title 47 of the Code of Federal Regulations. Three bands are authorized for such use: 902 through 928 MHz, 2400 through 2483.5 MHz, and 5725 through 5850 MHz. Currently there are a number of constraints on the operations of such systems including a maximum radiated power of 1 watt, and a limitation on spectrum spreading techniques to direct sequencing and frequency hopping.
In view of the above cited regulatory constraint and common sense, radio frequency networks of this type need to have low radiated power. Warehouse facilities are often concentrated in the same geographic area and a high level of radiated power increases the probability of intersystem interference. For the devices that operate in the shared portions of the electromagnetic spectrum, there is also a need to minimize the radiated power to avoid interference with other devices operating on the same frequencies. Additionally, a system that is designed for relatively high power output would require high radiated power from its roaming terminals, which are typically battery operated. This situation would lead to requirements for larger, heavier batteries or shorter battery life.
Spread spectrum technology has been known for a number of years. Generally speaking, a spread spectrum radio signal is one for which the information signal is dynamically spread over a relatively wide bandwidth as opposed to systems that communicate using conventional narrow band techniques with fixed carrier frequencies. Early development of spread spectrum techniques was principally in the field of military communications because of the opportunities that a spread spectrum system gives the designer to make it resistant to attempts to jam a transmission and its resistance to interception. With respect to the latter characteristic, typically military spread spectrum systems minimize the transmission of information that is needed and useable for synchronizing the receiver to the particular spectrum spreading technique employed at the transmitter. The receiver must have knowledge of the spectrum spreading technique employed by the transmitter in order to perform a complementary operation at the receiving end so that the information can be retrieved from the received signal.
In non-military commercial applications, such as networks of the type that may embody the present invention, the resistance to interception is normally not a concern. However, the resistance to jamming manifests itself as a resistance to destructive interference from other users of the spectrum including adjacent radio frequency networks. One fundamental advantage of spread spectrum systems are that multiple systems operating in the same bandwidth look like noise sources to their neighbors.
When the use of spread spectrum for this application was first allowed by the United States Federal Communications Commission, several manufacturers entered the field with direct sequence spread spectrum systems. A direct sequence system is relatively straight forward to implement. In a direct sequence system the transmitted data stream is multiplied (via an EXCLUSIVE OR function) with a pseudo random sequence, normally referred to as a chip code. The bit rate of the pseudo random sequence exceeds the bit rate of the data signal. The output from the EXCLUSIVE OR operation is provided to a phase shift keyed modulator and modulates a fixed frequency carrier. Since the pseudo random bit sequence has statistics that approximate noise, this has the effect of transmitting a signal which looks like broad band noise with only a relatively nominal power peak centered about the carrier frequency.
At the receiving end a correlator, which must be synchronized to the pseudo random bit sequence used at the transmitter, is used to reverse the process and reassemble the original data stream.
Part of what made direct sequence spread spectrum systems attractive is the relatively low cost and the ability to retrofit to certain aspects of existing designs. In particular, since such systems transmit on a fixed frequency carrier, only the front end processing of the digital data stream, i.e., multiplying by the chip code, need be changed. In other words prior art designs for fixed carrier frequency transmitters can continue to be used. The conventional wisdom among designers of radio frequency data transmission networks has been that the use of a frequency hopping technique for spreading the transmission spectrum would cause the systems to be complex and expensive. Furthermore, it was believed that it would be difficult to design battery operated terminals that were competitive with other conventional fixed carrier systems or direct sequence spread spectrum systems with respect to battery size/weight and battery life considerations.
While a direct sequence spread spectrum system gives some advantages as compared to narrow band FM systems, there are a number of inherent characteristics of direct sequence systems that offset the advantages gained from the spreading of the spectrum. In particular, typical applications for low power radio frequency data communications network are in relatively large facilities. Thus they typically employ multiple base stations that define different coverage cells. Since the coverage area for a 1 watt transmitter may typically be on the order of tens of thousands of square feet, many of these systems employ multiple base stations which operate in physically adjacent cells. The multiple base stations must be designed so that they do not interfere with each other.
With direct sequence techniques, the only practical approach to the design of multiple base station systems is to channelize the different base stations. Since the direct synthesis spread spectrum systems have a constant carrier frequency, the channelization defines a plurality of channels with sufficient inter-channel spectral spacing to avoid adjacent channel interference. There are thus a finite number of channels that are available for use in a given portion of the spectrum in which these devices can operate.
Channelized direct sequence spread spectrum (DSSS) systems are particularly susceptible to narrow band interference. Since, by regulatory constraint, these operate in shared portions of the spectrum (in the U.S.A.), there are a number of sources that can and sometimes do interfere with particular channels of a channelized DSSS system. This is particularly true in the 902 through 928 MHz band.
It must be kept in mind that regulatory constraint requires the low power communication device in a Business Radio System to operate without interfering with other users of this portion of the spectrum, but the converse is not true. Thus it is incumbent upon designers of radio frequency networks of this type to design the systems so that they do not interfere with other users of the spectrum. They also should minimize the susceptibility of their network to interference from existing sources, which are not constrained to pay them the same courtesy. Among the authorized uses of the 902 through 928 MHz spectrum in the United States of America are government radio location services, private operational fixed microwave systems, automatic vehicle monitoring systems, portions of the amateur radio services (ham radio), and anti-shoplifting devices used in retail establishments. Once a channelized DSSS system is installed, an authorized use of the spectrum that interferes with one or more of the channels of the DSSS system may be subsequently established in a neighboring area. This can lead to a breakdown of the operability of a system and may require the system operator to terminate the use of certain channels that the system was designed to use.
Additionally, in DSSS systems, the coverage area tends to decrease as the data rate increases, and the jamming margin decreases with data rate. As noted above, the resistance to jamming is one of the principle benefits of use of spread spectrum transmission techniques. The jamming margin is a figure of merit that relates to the resistance of the system to interference. The jamming margin is defined as G.sub.p -[L.sub.sys +[S/N].sub.out ] where G.sub.p is the processing gain of the system, L.sub.sys are the system losses, and [S/N].sub.out is the output signal to noise ratio. The process gain is a figure of merit indicative of the information throughput gained by employment of spread spectrum technique in use. In particular, use of spread spectrum techniques allows a system to be designed with a higher information rate under current regulations. The processing gained in a DSSS system is approximately equal to the ratio of the channel bit rate (i.e., the chip rate) to the information bit rate. Therefore, as the information bit rate rises, the ratio of this rate to the channel bit rate decreases and the jamming margin is lowered. In a frequency hopping spread spectrum system, the processing gain is directly related to the ratio of the total channel band width to the band width occupied during each hop.
Thus, while there are advantages to be gained from DSSS systems, they are more susceptible, as compared to a frequency hopping spread spectrum system, to interference. This is particularly true in the regulatory environment within the United States of America where licensed relatively high powered radiators operate in the same portions of the spectrum. It has been found that external narrow band licensed devices in the 902 through 928 MHz band can disable operation of one or more channels of a DSSS system from as far away as seventeen (17) miles.
Also, some of the inherent benefits of the use of existing fixed carrier frequency technology in the design of DSSS radio frequency networks is offset as multiple channel/multiple base station systems are implemented. This is because movable, i.e., roaming, terminals within the system must be designed to accommodate handing off between channels as they move out of the coverage area of one base station into the coverage area of another. Therefore, the benefit of the relative simplicity of the roaming terminal receiver circuitry is offset as the need to accommodate the reception of multiple channels on the same system arises.
A frequency hopping spread spectrum system (FHSS) typically has a lower data rate than a roughly equivalent DSSS. Both DSSS and FHSS systems have significantly higher data rates than prior art narrow band FM devices. For example, previous narrow band FM products of the assignee of this application provided 9600 bit per second data rates whereas the preferred embodiment of the invention described herein achieves 64 kilobits per second. It has been recognized for some period of time that frequency synthesizer design needed for a frequency hopping spread spectrum system is more complex and consumes more power than that required in a direct sequence direct spread spectrum system. While roaming terminals must be designed to accommodate different channels, they need only be designed so that they can make the transition from cell to cell in an appropriate manner and then remain on the channel for the cell in which they are operating until they move to the next adjacent cell. A frequency hopping system is constantly and rapidly changing the transmitted and received frequencies and this leads to both a more complex system and a higher power consumption.
Furthermore, in a frequency hopping system, the complex synthesizer design must be included in the transmitters and receivers at the base stations, thus increasing complexity and cost. As noted above, prior art spread spectrum radio frequency data communication networks were uniformly direct sequence systems. It is believed that this was in large part due to the belief that the complexity and power consumption that would be required for the roaming terminals would be such that they would have unacceptably short battery lives or would require bulky and heavy batteries that would make users reluctant to use such systems. Indeed, these problems were present and the inventors of the present invention have addressed and overcome them.
The spread spectrum transmission technique is accomplished at the physical layer of the system in use. Turning for a moment to the International Standards Organization (ISO) model of a communication systems, the media access layer of the system is the layer at which a protocol defines the rules for devices gaining access to the transmission medium. In other words, at the media access layer, a protocol must be in place that determines the rules for devices gaining access to the physical transmission medium, i.e., the spread spectrum transmitter and receiver in the case of a spread spectrum radio frequency data communications network. Prior art constant frequency fixed spectrum devices have often used a media access protocol referred to as slotted ALOHA. Slotted ALOHA is slotted in that it defines multiple time slots each of finite length. The slotted ALOHA protocol includes a rule that devices can only commence transmitting at the beginning of a time slot. The slots in slotted ALOHA are of sufficient duration to allow transmission of multiple packets of information. For relatively short messages, each slot is sufficient to allow transmission of an entire message. The slotting feature avoids certain situations where multiple collisions destroy all messages. In an unslotted system, it is possible for one devive to commence transmission and encounter a collision with a transmission from a second device. If, before completion of the second message, a third device starts transmitting, the system has encountered two successive collisions that have corrupted all three messages. By constraining devices only to start transmitting at the beginning of certain time slots, and employment of appropriate rules for delays of attempts to retransmit, a similar situation for the timing of the need to send messages at three remote devices will give rise to only a single collision.
Slotted ALOHA is a selected and widely used media access protocol. However, it does require broadcasting of a slot synchronizing signal so that all remote transmitting devices can synchronize their slot clocks to each other and that of a base station.
Therefore, it is desirable to employ a protocol similar to the slotted ALOHA media access protocol because it is effective at reducing the occurrence of collisions and the algorithmic design work implementing same has been done in the prior art. However, it is a nontrivial problem to be able to broadcast the slot synchronizing signal at a sufficient rate in a data communication system in which the physical layer employs a technique of spectrum spreading.
Given the jamming margin superiority of frequency hopping spread spectrum systems as compared to direct sequence spread spectrum systems, their increased coverage area and higher resistance to multipath fading, it is desirable to be able to employ a frequency hopping spread spectrum system in a radio frequency data communications network that overcomes both the cost and power consumption problems inherent in frequency hopping systems employing battery operated transmitters and to also use a media access protocol that requires a synchronized master clock source, such as slotted ALOHA.